On-power refueling

On-power refuelling provides the principal means for controlling reactivity in the CANDU 6. Additional reactivity control, independent of the safety shutdown systems, is achieved through use of reactivity control mechanisms. These include light-water zone compartments, absorber rods, and adjuster rods all are located between fiiel channels within the low pressure heavy water moderator and do not penetrate the heat transport system pressure boundary. The reactor is controlled by the dual redundant computer control system. The overall station control system is described in Section 5.7.2.3. 

On-power refuelling, to eliminate the need for refuelling outages... 

On-power refueling and a failed fuel detection system allow fuel that becomes defective in operation to be located and removed without shutting down the reactor. This reduces the radiation fields from released fission products, allows access to most of the containment while the reactor is operating and reduces operator doses. 

The HWR lattice features a long neutron migration length. This is a characteristic of the moderating properties of heavy water. It explains the relatively large HWR lattice pitch (28.6 cm for a CANDU reactor), which in turn makes possible the pressure tube design and the consequent capability for on-power refueling. 

The excellent neutron economy of the HWR stems from several design features necessitated by the original desire to use NUE fuel. Neutron absorption is minimized by the choice and strategic use of reactor structural materials, and by the use of heavy water as a moderator and coolant. In addition, on-power refueling minimizes the excess reactivity required to maintain criticality. This eliminates the need for any neutron-absorbing materials needed to suppress the initial excess reactivity associated with refueling a LWR core. 

On-power refueling contributes to HWR fuel-cycle flexibility by permitting very low excess core reactivity and core reactivity characteristics that change very little throughout the fuel cycle by replacing fuel on a daily or quasi-daily basis (see Section 15.5). 

Ultimately, bundles can be removed from the channel during on-power refueling and reshuffled, and reinserted in any order. This axial shuffling provides nearly unlimited capability for shaping the axial power distribution, if necessary. Adjuster rods are located interstitially between fuel channels, in the low-pressure moderator. They flatten the power distribution with NUE fuel, a function not required with enriched fuel, and provide xenon-override capability. With an enriched fuel, the adjuster rods can be easily replaced, if desired, or even eliminated, providing further flexibility in accommodating advanced fuel cycles. 

As a member of the CANDU family, the CANDU 300 design closely follows that of the larger CANDU 600 and CANDU 950 nuclear power plants and is is illustrated in Figure A key CANDU features include a pressure tube reactor, heavy water (D2O) moderator, natural uranium fuel, and on-power refuelling. 

This meeting was organized as a forum for experts of Member States to advise the IAEA on the different types of water and liquid metal cooled ship propulsion reactors, barge mounted power reactors and innovative reactor concepts which do not require on-site refuelling, and other similar reactor types presently in existence or under consideration in their countries. The purpose of the meeting was also to obtain advice from Member States on their needs and interests in the context of the IAEA s small and medium reactor programme. 

NEW CONCEPTS FOR SMALL POWER REACTORS WITHOUT ON-SITE REFUELLING FOR NON-PROLIFERATION... 

The infrastructure necessary to support conventional nuclear power development is very expensive, and beyond the resources of most developing countries. One of the primary goals of the proposed approach is to reduce the need for such an infrastructure. The system requirements of highly autonomous operation, simplified and minimized system maintenance, and elimination of all on-site refueling all significantly contribute to this goal. 

The elimination of on-site refueling directly attacks the two greatest proliferation risks of the traditional power reactor accessibility of materials and use of the facility for illicit purposes. Elimination of on-site refueling removes easy access to both fresh and spent fuel from the reactor site. Fissile material is found only inside the reactor, where it is protected by both limitations of physical access and a very intense inherent radiation barrier. The only period where fissile materials might be considered at risk is during transportation and set up, and during early operation where the fission product buildup is limited. Access to fissile materiids and use of the reactor for illicit irradiation is furdier complicated by the lack of physical features and infi astructure to open the reactor vessel. 

BROWN, N.W., HESSBERGER, J.A., "New concepts for small power reactors without on-site refueling for non-proliferation", IAEA-AG-1021IWGER/97, Obninsk, Russian Federation, July 1998. 

Successful resolution of such a problem requires a comprehensive systems approach diat considers all aspects of manufacturing, transportation, operation, and ultimate disposal. Some elements of this approach have been used previously in the development of propulsion (ship and space) nuclear power systems, with consideration given to many diverse requirements such as highly autonomous operation for a long period of time, no planned maint ance, no on-site refuelling and ultimate disposition. 

All UK nuclear power stations are designed for on-load refuelling, and safety during operation is important if the station is to continue to generate at full load over the days and weeks ahead. During operations close liaison between the Central Control Room (CCR), engineers and industrial staff is essential. 

Other training programmes on the simulator include reactor shutdown, on-load refuelling and turbine run-up, as well as abnormal or fault situations, including the loss of auxiliary plant with the reactor at power, such as ... 

The tour of the station included a visit to the Central Control Room, Cas Circulator House and Turbine Hall. The pile cap was viewed from the visitors observation gallery. At the time of the visit. Reactor 3 was at 30% power for on-load refuelling operations and Reactor 4 was at full power. 

In the case of the slightly enriched AGR reactor, on-stream refueling has been chosen for the first power plants (5). 

Alternately, semi-continuous on-power or batch refuelling can be accommodated. Preliminary calculations show that if the fuel blocks of each colunm coimected by a tie rod, are employed... 

Reload enrichment at the equilibrium (wt%) 0.711 wt% Refuelling frequency (months) 10 months gradual changeover of core with ave. refiielling rate of 15 fuel bundles/full power day Type of refiielling (on/off power) On-power Fraction of core withdrawn (%) 0.33 %/day Moderator material and inventory >99.75 wt.% D2O ... 

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